PHYS 195 chapter 5 (part two)

PHYS 195 chapter 5 (part two) - Frictional Force Friction...

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10/7/2009 Physics 201, UW-Madison 1 Frictional Force Frictional Force z Friction Í Opposes motion between systems in contact Í Parallel to the contact surface Í Depends on the force holding the surfaces together » Normal force (N) z Static friction Í Force resisting motion of a stationary object »f s is less than or equal to μ s N » If f > μ s N object starts to move z Kinetic friction Í Frictional force on an object in motion ± f k = μ k N
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10/7/2009 Physics 201, UW-Madison 2 Kinetic Friction Kinetic Friction z What does it do? Í It is in opposition to relative motion! z How do we characterize this in terms we have learned? Í Friction results in a force in the direction opposite to the direction of relative motion! m a F APPLIED f FRICTION m g N i j j ± f Friction = μ k N
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10/7/2009 Physics 201, UW-Madison 3 Crate Crate You are pushing a wooden crate across the floor at constant speed. You decide to turn the crate on end, reducing by half the surface area in contact with the floor. In the new orientation, to push the same crate across the same floor with the same speed, the force that you apply must be about a) four times as great b) twice as great c) equally as great d) half as great e) one-fourth as great as the force required before you changed the crate orientation. Frictional force does not depend on the area of contact. It depends only on the normal force and the coefficient of friction for the contact.
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10/7/2009 Physics 201, UW-Madison 4 Box Box z A = A B and material but with different masses, m A < m B are placed on the floor. The force needed to set box A in motion compared to that on box B is: A. larger B. the same C. smaller A B A B
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10/7/2009 Physics 201, UW-Madison 5 Example 1 Example 1 T=50 N M = 5 kg μ k = 0.2 Find acceleration of block Y: Normal force is balanced by gravity because there is no vertical motion, i. e., N = W = Mg X: Frictional force, f = μ k N= μ k Mg opposes the tension T Net force, F net = T-f Acceleration, a x = F net / M = (T- μ k Mg)/M= (50 - 0.2 x 5 x 9.8)/ 5) m/s 2 = 8.04 m/s 2 F x f T N W F y
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10/7/2009 Physics 201, UW-Madison
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This note was uploaded on 01/04/2011 for the course PHYS 195 taught by Professor Anderson during the Fall '07 term at San Diego State.

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PHYS 195 chapter 5 (part two) - Frictional Force Friction...

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